Coating compositions containing metal flake pigmentation are useful for the production of the currently popular “glamour metallic” finishes upon the surfaces of cellular phones, handheld electronic game and media entertainment devices, automobile interiors, computer housings, TV cabinets, furniture, automobile bodies, and the like. Such metal pigments include, for example, aluminum flake, copper bronze flake, metal oxide coated mica, and the like. Recently, there has been an effort in the coating industry to reduce atmospheric emissions of volatile solvents released during the painting process. One approach to this end has been to develop waterborne coating compositions. Unfortunately, many of the waterborne coating compositions containing metal flake pigments are unstable because some metal pigments react with the aqueous medium to produce hydrogen gas. Such “gassing” may cause unsafe pressure buildups in painting and storage equipment, and can adversely affect the appearance of the applied coating. For example, Aluminum flake pigment is widely known for its ability to impart metallic luster to coating compositions in which it is used. While the aluminum flake pigments have proven readily useful in coating compositions based on organic solvent systems, difficulties have been encountered in attempting to use the same pigments in aqueous coating systems (e.g., paints). In aqueous medium, aluminum flake pigment undergoes a reaction with water with a concomitant evolution of hydrogen gas. This gassing can be especially troublesome when the coating composition containing the pigment is stored in sealed containers. The reaction with water, on one hand, can reduce or destroy the desired optical effect of the pigment and, on the other hand, the hydrogen gas formed can create high pressures within storage tanks and composition containers, not to mention the explosion hazard.
Due to the increasing demand for aqueous systems, a number of techniques have been proposed for inhibiting or reducing the attack on the pigment flakes by water. One technique involves encapsulating the aluminmum pigment with dense amorphous silica. Other techniques involve treating the pigment with vanadate, chromate, molybdate or organophosphate passivating compounds. However, coating, encapsulating, or passivating the metal pigment with the various coatings, encapulants and reagents above and those outlined below can deleteriously affect the optical properties of the pigment to such an extent as to render it undesirable for finishes requiring metallic luster or sheen. Additionally, when these treatments are used in large amounts to treat metal pigment particles in a coating, they can negatively affect other properties of the coating such as the coating's ability to adhere to a surface.
Compositions relating to improved stability of metal pigments in waterborne coatings includes the following:
U.S. Pat. No. 4,717,424 relates to the surface treatment of metal pigments with carboxy-alkylene phosphoric acid ester or carboxyl-alkaline phosphonic or phosphinic acid to improve the stability of metal pigments in aqueous coatings.
U.S. Pat. No. 5,104,922 relates to phosphated acrylic polymers made in solution polymerization to improve stability of aqueous aluminum flake dispersion.
U.S. Pat. No. 5,151,125 relates to the use of low molecular weight polymer containing a phosphate monomer to improve stability of metallic pigments.
U.S. Pat. No. 5,320,673 relates to the use of a dispersant with silane or phosphate frunctional groups made in solution polymerization to improve stability of metallic pigments in waterborne coating compositions.
U.S. Pat. No. 5,356,469 relates to the use of a heteropoly anion compound and a phosphosilicate to improve stability of metal pigment aqueous paste.
U.S. Pat. No. 5,540,768 relates to the use of molybdic acid and a phosphoric eater to stabilize aluminum pigments in water base metallic paints.
U.S. Pat. No. 5,755,869 relates to the use of fatty acid or a maleinized alpha-olefin modified gassing inhibitor to treat the surface of metal pigments and improve their stability in waterborne film-forming compositions.
U.S. Pat. No. 6,624,227 relates to the use of phosphonic acid reaction products as additives to improve stability of metallic pigments in aqueous coatings.
U.S. Pat. No. 6,485,786 relates to the use of phosphorus containing unsaturated monomers in emulsion polymers to improve the stain-blocking in aqueous coatings.
U.S. Pat. No. 6,710,161 relates to the use of phosphorus containing unsaturated monomers in emulsion polymers for aqueous metal coatings. U.S. Pat. No. 6,765,459 relates to the use of phosphorus containing unsaturated monomers in emulsion polymers to improve gloss and corrosion resistance in waterborne coatings.